1. Field of the Invention
The present invention relates to electronic sensors, and more particularly to radar motion sensors based on pulse echo range gating.
2. Description of Related Art
Continuous Wave CW Doppler radar motion sensors emit a continuous wave, radio frequency RF carrier and mix the transmitted RF carrier with the return echos to produce a difference frequency equal to the Doppler shift produced by a moving target from which the carrier is reflected. CW Doppler sensors have a number of deficiencies that limit their wide spread application, including: 1) lack of a definite range limit, which leads to false triggers on distant clutter; 2) extremely high sensitivity at close range, causing false triggering on nearby small objects likes insects and on vibration; 3) high current consumption due to the CW operation making battery operation impractical; and 4) inability to co-locate sensors due to mutual interference.
Range limiting or gating of the Doppler radar can be implemented with either amplitude modulation AM or frequency modulation FM. In U.S. Pat. No. 3,719,944 by Adrian, a range limited FM Doppler proximity fuzing system for the U.S. Navy is disclosed. In this system, the range limit is related to FM bandwidth, and for short ranges of for example less than 3 meters, a high FM bandwidth, for example greater than 100 megaHertz is required. A main shortcoming with FM Doppler is response lobes that extend beyond the range limit. In other words, the range limit is not an absolute cut-off, just a substantial reduction in response. Another deficiency is the lack of range precision in low cost implementations. FM deviation in microstrip type oscillators is not well defined. In spite of these limitations, FM Doppler provides a response amplitude versus target distance that is much more uniform than seen in CW Doppler. The Adrian patent describes this technology in detail.
AM, or pulse, Doppler motion sensors have been described in U.S. Pat. No. 4,197,537 by Follen, et al., in U.S. Pat. No. 5,682,164 by McEwan, and others. In pulse Doppler, a short pulse is transmitted and its echo is mixed with either the transmitted pulse or a local oscillator such that either the pulse timing or its width defines the range gated region. With amplitude modulation, response beyond the gated region is zero because there is no leakage as in FM Doppler. While pulse Doppler exhibits excellent gating characteristics, its voltage response versus range varies with 1/R.sup.2, where R is equal to the range to the target. This 1/R.sup.2 characteristic occurs with CW Doppler as well. Thus a target at 10 meters range and a small object like an insect at 1 centimeter range may produce the same response. This problem with small objects and with nearby vibrations makes radar motion sensing too unreliable for many applications.
AM (pulse) and FM Doppler have been combined advantageously by Tresselt, U.S. Pat. No. 3,898,655, and by McEwan, U.S. Pat. No. 5,521,600. Pulse FM Doppler combines the best of both modulation techniques, resulting in zero response beyond range cutoff, and more uniform response within the gated region. The limitations with pulse FM Doppler include 1) difficulty in accurately controlling FM deviation, 2) an overly wide spectrum due to the combination of modulations, leading to regulatory constraints with for example the Federal Communication Commission FCC, and 3) the response within the gated region tends to vary with 1/R.
Accordingly there is a need for an improved radar motion sensor technology for commercial uses, and particularly for low power and short range applications.